1. Field of the Invention
The present-invention relates to an optical switching system that uses micro mirrors such as MEMS (Micro Electro Mechanical Systems) mirrors for switching optical signals entering from an input port and sending out a selected optical signal and a control method for the micro mirrors in the optical switching system. For example, the present invention can be used for an OXC (Optical Cross-Connect) system or the like of WDM (Wavelength Division Multiplexing) communication, space division communication or the like.
2. Description of the Prior Art
Recently, a micro machine device has become a focus of attention, which has a micro structure obtained through a process of micro machine processing technique (also referred to as MEMS [Micro Electro Mechanical Systems] or MST [Micro System Technology]). As one of the micro machine devices, a micro mirror (or a DMD [Digital Micro-mirror Device]) is developed and used for an optical switching system that is installed in a node of an optical network.
The optical switching system includes a plurality of micro mirrors having reflection planes whose angles can be controlled and that are arranged on a plane. Optical signals that enter a plurality of input ports are reflected by the plurality of micro mirrors and are led to selected corresponding output ports among a plurality of output ports. Since the micro mirrors are controlled and the ports are switched in this way, it is possible to perform optical exchange of a plurality of channels of optical signals on an optical transmission path.
However, since the optical system of the optical switching system including lenses and the micro mirrors has a three-dimensional structure, optical transmission characteristics alter largely in accordance with expansion or contraction due to variation of temperature. As a result, an optical coupling loss may vary largely.
Conventionally, in order to reduce influence of such temperature variation, there is proposed a method of compensating a temperature drift component by feedback control so that an optimal driving state is maintained (see U.S. patent application publication No. US2004/0037490).
In addition, U.S. Pat. No. 7,155,125 proposes a method in which a memory portion for storing control information about a tilt angle of each micro mirror corresponding to setting of an optical path with respect to a specific reference temperature and a temperature sensor for measuring temperature of an optical switch are provided, and when request for connecting an optical path is issued, control information corresponding to the request is read out from the memory portion, drive voltage compensated with deviation due to temperature variation to the reference temperature is calculated based on temperature measured by the temperature sensor, and the calculated drive voltage is used as an initial value.
However, in the case of the method in which the feedback control is performed as described in U.S. patent application publication No. US2004/0037490, although the temperature variation can be compensated securely, there is a problem that the circuit structure becomes complicated, and the circuit element or the circuit board is required to have a large mounting area.
In addition, in the case of the method in which the deviation due to temperature variation from the reference temperature is determined by calculation as proposed in U.S. Pat. No. 7,155,125, although the circuit structure becomes simplified, there is a problem that if the temperature variation from the reference temperature is large, an error thereof is large. In particular, influence due to temperature variation is large when loss adjustment is performed by using attenuation of VOA (Variable Optical Attenuator). When this attenuation is used, there is a problem that it is difficult to perform the compensation with high accuracy, and accuracy is lowered easily.
In addition, there is a large difference between the temperature time constant of the temperature sensor and a temperature time constant of a switch fabric that is a structural element of the optical switching system, so this difference may cause a large error in the control.